1. Field of the Invention
The present invention relates to a transmissive display apparatus such as an HUD (head-up display) which projects a video picture onto a transparent display panel and causes a user to simultaneously view a virtual image thereof and a transparent background, a mobile object provided with such a transmissive display apparatus, and a control apparatus which controls an image display unit.
2. Description of the Background Art
Conventionally, a transmissive display apparatus which projects a video picture onto a transparent display panel and enables the simultaneous viewing of a virtual image thereof and a transparent background that can be viewed through the transparent display panel has been proposed. In particular, a so-called head-up display (HUD) which projects and displays a video picture on a windshield in order to indicate the speed and various warnings in the field of front vision during the driving of an automobile and the like is being developed.
When this kind of transmissive display apparatus is used, since the driver can view information (for instance, a map or speedometer) related to driving while simultaneously viewing the front external environment, the driver can drive more safely.
FIG. 10 shows an example of a conventional HUD. In this example, the simultaneous viewing of the front external environment and the display light is enabled by reflecting the display light at the windshield portion toward the driver.
In FIG. 10, 101 denotes a vehicle body that is equipped with the HUD. 102 denotes an HUD optical unit that is housed inside the dashboard, and internally includes a display unit 103 and a deflection unit 104. The display unit 103 is configured, for example, with a liquid crystal element and a light source, and displays information to be displayed to the driver's viewpoint 107. The display light that is displayed by the display unit 103 is projected toward the deflection unit 104. The deflection unit 104 is configured with a mirror or the like, and deflects the display light from the display unit 103 toward a display area 105 on a windshield 106. The display area 105 of the windshield 106 reflects the display light from the HUD optical unit 102 toward the driver's viewpoint 107, and forms a virtual image 108 of the display image. The driver can visually check information related to driving by viewing the display light that was reflected by the display area 105 of the windshield 106.
With respect to the visible light transmittance of the windshield 106, the perpendicular transmittance of the visible light is defined in order to maintain the field of view of the external environment. For example, the Japanese “Safety Standards for Road Transport Vehicles” prescribes this as 70% or higher. Accordingly, the reflectance of the display area 105 is restricted so that the perpendicular transmittance of the visible light is at least a defined value or higher. Normally, the surface reflection of glass changes depending on the incidence angle or deflecting direction. Although it is approximately 4% with vertical incidence, when the inclination of the windshield 106 often used in passenger cars is roughly 30°, in order to project the display light to the position of the viewpoint 107, the incidence will be 60 to 70° and the reflectance will be roughly 10 to 12%.
A double image is generated in this kind of conventional HUD that reflects the display light with the windshield 106.
FIG. 11 is a diagram showing the configuration of the windshield 106. With the windshield 106, normally, an inner glass 202 and an outer glass 203 are bonded via an intermediate film 201. An incident light 204 that is emitted from a certain pixel point P of the display unit 103 and enters the windshield 106 is reflected at a point A11 of the front face of the inner glass 202, becomes a front face reflected light 205, and thereby generates a display image point 207. Moreover, the incident light 204 is reflected at a point B11 of the rear face of the outer glass 203, becomes a rear face reflected light 206, and generates a ghost image point 208.
Accordingly, since there are two light paths that reach the viewpoint 107, the video picture that is projected from the display unit 103 will not be a clear video picture as shown in FIG. 12A, and will be viewed as a double image that overlaps vertically as shown in FIG. 12B.
Several methods have been proposed for reducing this kind of double image.
For example, there is a method of providing an optical interference film on the front face of the windshield (Japanese Patent Application Publication No. S60-124532). By using a filter which reflects light having a wavelength of a specific range at a high reflectance and transmits light of other wavelengths at a high transmittance, the light is adjusted to the wavelength spectrum of the display light that is emitted from the display unit, and it is thereby possible to increase the reflectance of the display light while ensuring a high transmittance. If the reflectance of the display light is high, the quantity of light that reaches the viewpoint as the rear face reflected light will decrease by that much, and the double image can thereby be reduced.
Moreover, there is a method of converting a display light into an S-polarized light and causing it to enter the windshield at a Brewster's angle (Japanese Patent Application Publication No. H2-141720). As a result of using the nature of the Brewster's angle in which the reflectance of a P-polarized light becomes zero and the reflected light completely becomes an S-polarized light, the transmitted S-polarized light is rotated to the P-polarized light with a wave plate or the like provided inside the windshield, and the rear face reflected light becomes the P-polarized light so as to eliminate any reflection and prevent a double image. The Brewster's angle is an angle that is determined by the refractive index, and is approximately 56° with glass having a refractive index of 1.5.
Moreover, there is also a method of providing a hologram within the windshield, and diffracting the display light transmitted through the front face so as to prevent the rear face reflected light from reaching the viewpoint (Japanese Patent Publication No. 2751436). If the diffraction efficiency of the hologram is designed to be approximately 100% and a sufficiently large diffraction angle can be attained, a double image can be effectively prevented.